Evaluation of Sodium Relaxation Times and Concentrations in the Achilles Tendon Using MRI

Abstract

Sodium magnetic resonance imaging (MRI) can be used to evaluate the change in the proteoglycan content in Achilles tendons (ATs) of patients with different AT pathologies by measuring the ²³Na signal-to-noise ratio (SNR). As ²³Na SNR alone is difficult to compare between different studies, because of the high influence of hardware configurations and sequence settings on the SNR, we further set out to measure the apparent tissue sodium content (aTSC) in the AT as a better comparable parameter. Ten healthy controls and one patient with tendinopathy in the AT were examined using a clinical 3 Tesla (T) MRI scanner in conjunction with a dual tuned ¹H/²³Na surface coil to measure ²³Na SNR and aTSC in their ATs. ²³Na T₁ and T₂* of the AT were also measured for three controls to correct for different relaxation behavior. The results were as follows: ²³Na SNR = 11.7 ± 2.2, aTSC = 82.2 ± 13.9 mM, ²³Na T₁ = 20.4 ± 2.4 ms, ²³Na T_2s* = 1.4 ± 0.4 ms, and ²³Na T_2l* = 13.9 ± 0.8 ms for the whole AT of healthy controls with significant regional differences. These are the first reported aTSCs and ²³Na relaxation times for the AT using sodium MRI and may serve for future comparability in different studies regarding examinations of diseased ATs with sodium MRI.

Keywords: 23Na MRI; Achilles tendon; collagen; glycosaminoglycan; proteoglycan; sodium MRI; sodium concentration; sodium relaxation time; tendon.

Figure 1

Data points and fitting results of ²³Na T₁ relaxation times in the different Achilles tendon (AT) regions of interest (ROIs) of an exemplary healthy control. ROIs were named after their position, where (a) corresponded to the most distal 3 cm of the AT, i.e., the distal 3 cm of the tendon’s length extending into the calcaneal insertion (INS), (b) corresponded to the middle 3 cm, i.e., the middle portion of the AT (MID), and (c) corresponded to the proximal 3 cm, which was referred to as the myotendinous junction (MTJ). In (d), the data and fitting of the total ROI combining all three ROIs, i.e., INS, MID and MTJ are displayed. The corresponding parameter results were (a) ²³Na T₁ = 21.2 ms, R² = 0.999, (b) ²³Na T₁ = 22.1 ms, R² = 0.999, (c) ²³Na T₁ = 19.6 ms, R² = 0.997 and (d) ²³Na T₁ = 23.1 ms, R² = 0.999.

Figure 2

Data points and fitting results to determine ²³Na T₂* relaxation times in the different AT ROIs of an exemplary healthy control. The corresponding parameter results were (a) ²³Na T_2s* = 1.0 ms, ²³Na T_2l* = 13.3 ms, p_s = 32.7%, R² = 0.997, (b) ²³Na T_2s* = 1.2 ms, ²³Na T_2l* = 13.6 ms, p_s = 34.9%, R² = 0.998, (c) ²³Na T_2s* = 1.4 ms, ²³Na T_2l* = 13.8 ms, p_s = 35.7%, R² = 0.994 and (d) ²³Na T_2s* = 1.1 ms, ²³Na T_2l* = 13.3 ms, p_s = 33.2%, R² = 0.998.

Figure 3

Apparent tissue sodium concentration (aTSC) (a,d) and ²³Na signal-to-noise ratio (SNR) (b,e) as well as ¹H T₂* maps (c,f) overlaid onto the ¹H images acquired with the density-adapted radial sequence using TE = 9 ms. The overlaid colormaps are displayed for an exemplary healthy control (a–c) and the patient with Achilles tendinopathy (d–f). The different AT ROIs (INS, MID and MTJ) are outlined and labelled according to their position.

Figure 4

Sagittal proton density (PD) (a,c) and T₁ (b,d) weighted images of an exemplary healthy control (a,b) and the patient with Achilles tendinopathy (c,d) acquired as anatomical reference in line with clinical standard procedures. The patient showed local thickening mainly in the middle part of the AT with adjacent peritendinous fluid in the same region, which is both highlighted in (c,d) with white arrows.

Figure 5

Boxplots illustrating the results for healthy controls for (a) aTSC, (b) ²³Na SNR and (c) ¹H T₂* for the AT ROIs INS, MID, MTJ and the total ROI, which is the combination of the previous three. The parentheses above the boxplots indicate whether the difference between the groups was significant (*) in the Bonferroni corrected Wilcoxon rank-sum tests with a significance level of p ≤ 0.05.